The frequency of the output of a laser can be important to its use and functioning. By way of example, currently, the telecommunications industry uses frequencies in the 1550 m range. Indeed, specific frequency outputs are often needed for both scientific research as well as commercial applications. As such, much time and effort has been spent attempting to utilize the stimulated Raman scattering effect (SRS) to produce lasers having a particular frequency output. The present invention describes a new implementation of SRS, interchangeably referred to herein as a Raman laser, and related methods. The SRS is a result of the scattering of a light photon by a molecule into a lower energy photon, with the resulting energy going into vibration of the molecule. This results in a shift of the input light beam to a longer wavelength, where the shift is dependent on the Raman gain medium used for implementing the SRS and the pump wavelength.
SRS sources may be helpful in extending the available range of semiconductor lasers, particularly to hard to access frequencies. Raman lasers have been made in a number of geometries, ranging from standard laser cavities made up of a Fabry-Perot cavity (similar to two flat mirrors and a Raman gain medium), to cavities based on a length of optical fiber with Bragg “mirros” (reflectors made in an optical fiber). The Raman effect is a nonlinear optic effect, i.e., the strength of the effect scales with the intensity of the input light wave. As such, efficient generation of Raman light requires very high pump power, and is typically generated using macroscale devices.
A discussion of some of the efforts in this area can be found in U.S. Patent Application publication number US21010696A1, published on Aug. 1, 2001, the contents of which are incorporated herein by reference. Early work in this area, for example, Qian, S.X., Chang, R. K. Multiorder Stokes Emission from Micrometer-Size Droplets. Phys. Rev. Lett. 56, 926-929(1986); and, Lin, H. B., Huston, A. L. Eversole, J. D., Campillo, A. J. Double-resonance stimulated Raman-scattering in micrometer-sized droplets. J. Opt. Soc. Am. B 7,2079-2089 (1990), the disclosures of which are incorporated herein be reference, also relied on the use of Raman excitation in mircodroplets—but microdroplets have not been found to be useful in practical applications. Thus, there is a need fro a self-contained device and related methods which can produce a lased output in a frequency range of interest.
It is known to one of skill in the art how to couple a waveguide to an optical resonator so as to transfer optical power to the resonator from the waveguide or from the waveguide to the resonator. It is also known to one of skill in the art that power circulates in a resonator preferentially at resonant frequencies corresponding to optical modes of the resonator. Likewise the principles associated with the use of mirco-resonators and transverse optical coupling through a fiber taper are understood to one of skill in the art. The following references provide additional information on these and certain related issues, the disclosure of each of which is incorporated by reference herein as if set forth in full hereat: Laine J P, Little B E, Lim D R, et al., Planar integrated wavelength-drop device based on pedestal antiresonant reflecting waveguides and high-Q silica microspheres, OPT LETT 25 (22): 1636-1638 Nov. 15, 2000; Laine J P, Little B E, Lim D R, et al., Microsphere resonator mode characterization by pedestal anti-resonant reflecting waveguide coupler, IEEE PHOTONIC TECH L 12 (8): 1004-1006 August 2000; Yariv, et. al., U.S. patent application Ser. No. 09/454,719, for “Resonant optical wave power control devices and method”, filed on Dec. 7, 1999, the contents of which are incorporated herein in full by reference.
Other references of interest include: Vahala, et. al, Micro-Cavity Laser, U.S. patent application Ser. No. 09/802,442, Filed on Mar. 9, 2001; Collot, L., Lefevre-Seguin, V., Brune, M., Raimond, J. M., Haroche, S. Very High-Q Whispering-Gallery Mode Resonances Observed on Fused Silica Microspheres. Europhys. Lett. 23, 327-334 (1993); Braunstein, D., Khazanov, A. M., Koganov, G. A., Shuker, R. Lowering of threshold conditions for nonlinear effects in a microsphere. Phys. Rev. A 53, 3565-3572 (1996); Knight, J. C., Cheung, G., Jacques, F., Birks, T. A. Phase-matched excitation of whispering-gallery-mode resonances by a fiber taper. Optics Letters 22, 1129-1131 (1997); Chang, R. K., Campillo, A. J. (ed.) Optical Processes in Microcavities (World Scientific, Singapore, 1996); Gorodetsky, M. L., Savchenkov, A. A., Ilchenko, V. S. Ultimate Q of optical microsphere resonators. Optics Letters 21, 453-455 (1996); Weiss, D. S. et al. Splitting of high-Q Mie modes induced by light backscattering in silica microspheres. Optics Letters 20, 1835-1837 (1995); Lai, H. M., Leung, P. T., Young, K., Barber, P. W., Hill, S. C. Time-independent perturbation for leaking electromagnetic modes in open systems with application to resonances in microdroplets. Phys. Rev. A 41, 5187-5198 (1990); Zhang, J. Z., Chang, R. K. Generation and Suppression of Stimulated Brillouin Scattering in Single Liquid Droplets. J. Opt. Soc. Am. B 6, 151-153 (1989); Cai, M., Painter, O., Vahala, K. J. Observation of Critical Coupling in a Fiber Taper to a Silica-Microsphere Whispering-Gallery Mode System. Phys. Rev. Lett. 85, 74-77 (2000); Lin, H. B., Campillo, A. J. CW Nonlinear Optics in Droplet Microcavities Displaying Enhanced Gain. Phys. Rev. Lett. 73, 2440-2443 (1994); Ilchenko, V. S., Gorodetskii, M. L. Thermal Nonlinear Effects in Optical Whispering Gallery Microresonators. Laser Physics 2, 1004-1009 (1992); Vemooy, D. W., Ilchenko, V. S., Mabuchi, H., Steed, E. W., Kimble, H. J. High-Q measurements of fused-silica microspheres in the near infrared. Optics Letters 23, 247-249 (1998); Bachor, H.-A., Levenson, M. D., Walls, D. F., Perlmutter, S. H., Shelby, R. M. Quantum nondemolition measurements in an optical-fiber ring resonator. Phys. Rev. A 38, 180-190 (1988); Silberhom, Ch., Lam, P. K., Weiss, O., Koenig, F., Korolkova, N., Leuchs, G. Generation of Continuous Variable Einstein-Podolsky-Rosen Entanglement via the Kerr Nonlinearity in an Optical Fiber. Phys. Rev. Lett. 86, 4267-4270 (2001); Treussart, F. et al. Evidence of the intrinsic Kerr bistability of high-Q microsphere resonators in superfluid helium. Eur. Phys. J. D 1. 235-238 (1998); Fan, X., Palinginis, P., Lacey, S., Wang, H., Lonergan, M. C. Coupling semiconductor nanocrystals to a fused-silica microsphere: a quantum-dot microcavity with extremely high Q factors. Opt. Lett. 25, 1600-1602 (2000)—the disclosures of each of the foregoing are incorporated herein by reference as if set forth in full hereat.)
Finally, there is a hydrogen/iodine-gas filled fabry-perot cavity Raman laser reported in the literature. See, e.g., Brasseur J K, Teehan R F, Knize R J, et al., Phase and frequency stabilization of a pump laser to a raman active resonator, IEEE J QUANTUM ELECT 37 (8): 1075-1083 August 2001; and Meng L S, Roos P A, Repasky K S, et al., High-conversion-efficiency, diode-pumped continuous-wave Raman laser, OPT LETT 26 (7): 426-428 Apr. 1, 2001.
One of the many disadvantages of the approaches of the prior art is that the Raman wavelength shift is very narrow, i.e. it only attains a very specific shift with a given pump frequency, whereas the present invention possesses a much broader gain spectrum, thus allowing tunablity by controlling resonator geometry. Additionally, the devices are often quite complicated.
Limitations of these and other devices include a limited ability to obtain desired output frequencies, high threshold power requirements, low emission and coupling efficiencies and large and/or highly complicated and expensive devices.
The preferred embodiment of the present invention overcomes these and the other limitations of the prior art by providing a compact, self-contained Raman laser source that, in the preferred embodiment, is directly coupled to an optical fiber waveguide. Indeed, the present invention can be entirely constructed from ordinary optical fiber. Optical fibers, in addition to being very important in modem optical communications systems, provide a very convenient means to convey both optical pump power to the laser and to convey emitted laser radiation from a Raman laser resonator. The ability to utilize stimulated Raman scattering effect to produce a Raman Laser output of a desired frequency output which is directly coupled to an optical fiber is therefore of great practical significance.